Additional info: http://www.nytimes.com/2013/07/27/arts/design/walter-de-maria-artist-on-grand-scale-dies-at-77.html?pagewanted=all&_r=0
Magazine work by de Maria.
List of showings of Hard Core (1969).
Mention of a drum duel between Walter de Maria and Max Neuhaus.
Announcement of the Sixth Guggenheim International Exhibition held February – April, 1971. Includes list of participating artists.